1. Field of the Invention
The present invention relates to a method of optimizing the operation points of lasers, and also to means for this end. More, specifically, the invention relates to a method of optimization that does not require the aid of external equipment as a reference.
2. Description of the Related Art
Tuneable semiconductor lasers include several different sections through which current is injected. The lasers typically have three or more sections. The wavelength, power and mode purity of the lasers can be controlled, by adjusting the current injected into the various sections. Mode purity implies that the laser shall be tuned to an operation point, i.e. tuned to a combination of the three or four injected drive currents, which is characterized in that the laser is distanced from a combination of the drive currents where so-called mode jumps take place and where lasering is stable and where sidemode suppression is high.
Special wavelength controls are required with different applications. For instance, in the case of sensor applications it must be possible to tune the laser continuously, so as to avoid mode jumps as far as possible In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over very long periods of time, after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer while a typical time period is twenty years.
In order to be able to control the laser, it is necessary to map the behavoir of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture. However, it is also necessary to discern degradation of a laser in operation and to be able to compensate for this degradation by changing the drive currents. A change in the wavelength for a given operation point is an example of such degradation.
Mapping of the behavoir of a laser is normally effected by connecting the laser to different measuring instruments and then varying the drive currents systematically. Such instruments are normally power meters, optical spectrum analyzers for measuring wavelength and sidemode suppression, and line width measuring devices. This laser measuring process enables all of these parameters to be fully mapped as a function of all different drive currents.
However, the process has decisive drawbacks. The measuring processes are highly time-consuming and also generate large quantities of unnecessary data. It is not until the laser has been measured-up that suitable areas of operation can be discerned. Furthermore, an additional wavelength reference, for instance an optical spectrum analyzer or wavelength measuring device, is required in order to carry out the measurements. This makes it difficult to subsequently check or adjust the setting and/or the calibration of lasers that have already been installed in operation.
The present invention eliminates these drawbacks.
The present invention thus relates to a method of optimizing the operation point of a laser, by characterizing the laser and by controlling the various sections of the said laser. The different sections of the laser are controlled by varying the injected currents; and the laser is sensed with respect to discontinuities occurring in mode jumps in a signal delivered by a sensing device firmly connected to the laser. Control of the various laser sections is effected with the aid of a control unit and delete different control combinations and the signal delivered by, the sensing device are sent to the control unit, which detects the mode plane of the laser. At least a part of a mode plane or several mode planes is/are stored in a memory belonging to the control unit, which is caused to control different laser sections such that the laser will obtain the desired operation point.
The invention also relates to apparatus for carrying out the method.